What an unfortunate situation you:re in. It sounds like the dealer is more concerned with money than customer satisfaction. They should be doing more to earn your trust. Two flawed machines right out of the box is unacceptable and uncommon.
I would approach the owner (not a regular employee) face to face and ask him (her) and tell them you really wanted that machine and bought it twice. And twice you have had tension issues that they admit to and is it unreasonable to not want to risk a third failed attempt. Ask if they- in your shoes- wouldn’t be equally baffled and disappointed. Tell them you really want to buy from them, but a different machine with a store credit for the full amount to paid them originally. Try to ask them questions (calmly but directly) to help them want to work together toward a fair solution. Let them know you’ve contacted Juki corporate to report your trouble with the two machines. Say “will you honor the price you sold it to me for since you can’t get the machine to work right?” Be likeable, reasonable, keep it about the faulty machine, your unwillingness to make the same mistake three times, and about them honoring their deal. You have not gotten a fair deal. You’ve lost the use of it for repair time, you ve lost confidence in this model but you don’t also want to lose the business relationship. Appeal to their reasonableness with logic, not emotion. Let Juki corporate try to yank on them first.